This invention relates generally to the production of purified aqueous solutions. More particularly, the present invention relates to the electrochemical reduction of oxychlorine species in aqueous solutions to remove such species by reducing them to environmentally safe chloride ions. The process and the apparatus employing the process are suitable for both waste water and potable water treatment. The electrochemical reduction occurs on a high surface area cathode structure. Both dilute and concentrated solutions of oxychlorine species can be reduced to environmentally safe chlorides by the process and apparatus of this invention.
Chlorine dioxide is widely used as an oxidizer and disinfectant for taste and odor control in drinking water and as a bleaching agent in the production of pulp and paper. It is also used for the oxidation of trihalomethane precursors in drinking water. Most of the chlorine dioxide generators used in drinking water treatment employ a 95-98% efficient chemical reaction between chlorine gas and sodium chlorite. The unreacted sodium chlorite remains as an oxyhalide that is increasingly undesirable both toxicologically and environmentally in drinking water.
In the commercial environment of pulp and paper mills, large volumes of chlorine dioxide are generated from the combined reaction of a chlorate salt, acid, and a reducing agent. Alkali metal or alkaline earth chlorate salts are employed with the typical being sodium chlorate. Common acids used in the process are sulfuric or hydrochloric. Representative reducing agents used are sodium chloride, methanol or sulfur dioxide. These reagents are used in various combinations depending upon the specific chlorine dioxide process employed. The aqueous flow streams from these processes all have the potential to contain significant amounts of unreacted chlorate, as well as chlorite and chlorate by-products produced from pulp and paper bleaching processes.
In drinking water applications, the by-products of chlorine dioxide treatment pose a major problem. These by-products are chlorite and chlorate. Chlorite is the principal by-product produced from the reaction of trihalomethane precursors and chlorine dioxide. Chlorate, once formed either from the chlorine dioxide generator or from by-product reactions, is not easily removed chemically. Previous approaches attempted to remove chlorite from drinking water by using sulfur-based reducing agents, such as sulfur dioxide, sodium bisulfite, and sodium sulfite. Unfortunately, these reducing agents produce chlorate as a major by-product of complex side reactions with oxygen in water. Another approach has used sodium thiosulfate as a reducing agent which does not produce significant amounts of by-product chlorate. Thiosulfate, however, suffers from safety considerations because of unwanted sulfur-oxygen by-products produced in the water.
These and other problems are solved in the process and by the apparatus employing the process of the present invention by removing both chlorite and chlorate from aqueous solutions, as well as other oxychlorine species, from aqueous solutions by the use an electrochemical reduction process.